Fix start & end of segment not "flowing" in Flow FX

[DedeHai]

start and end of segment were just showing the palette on several pixels, depending on segment size / zone width.
By adding two extra zones it now flows everywhere and the segment.fill() is no longer needed to cover up the integer math truncation.

Summary by CodeRabbit

• Bug Fixes
  • Improved the flow animation effect with refined zone distribution calculations for better segment coverage handling. This may result in visual adjustments to how the effect displays.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

Modified the mode_flow function in FX.cpp by converting several unsigned variables to signed integers (zones, zoneLen, offset). Updated loop variables and iteration logic. Adjusted zone calculations to add two extra zones and recompute offset to center zones, with corresponding index and casting adjustments.

Changes

Cohort / File(s)	Summary
Mode Flow Function Refactoring wled00/FX.cpp	Changed variable types from unsigned to signed int for zones, zoneLen, and offset; updated loop variables and iteration logic; modified zone calculation to add two extra zones and recenter offset; adjusted colorIndex and led calculations to align with new signed indices.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Suggested reviewers

• blazoncek
• netmindz

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix start & end of segment not "flowing" in Flow FX' directly addresses the main change: fixing an issue where segment start and end pixels weren't flowing properly in the Flow FX mode.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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